This invention relates to a mechanical analyzer, and, more particularly, to a thermomechanical analyzer for ascertaining certain physical and chemical characteristics of materials.
In the manufacture and use of materials, particularly organic materials, it is necessary to obtain such information as the materials softening temperature, tensile modulus, shrink temperatures, glass transition temperature, elastic modulus, and the like. Accordingly, it has become customary to ascertain this information and characterize various materials by the results of various mechanical property measurements including thermomechanical analysis. Measurements of this type find wide use, for example, on polymeric materials including fibers, yarns and films. In one instance, by measuring the relationship between temperature and the retractive force exerted by a fiber (and the resulting stress or tension if the fiber's cross-section area is known) one can obtain an indication of the structural changes occurring in the fibers as a function of temperature. Such property controls the use to which a fiber can be put. In the case of acrylic yarns, for example, the orientation and state of internal stress in the typing molecules tend to control the physical properties of the yarn itself.
The glass transition temperature of a polymer fiber is another property that is particularly useful in characterizing the fiber. Such temperature is useful in determining the conditions under which drawing, texturing, annealling, dying, fiber softening, crystallization and other properties can take place. Such temperature may be used in the prediction of the dyeability of the fiber.
A conventional means for measuring shrinkage tension, which is the retractive force exhibited, of yarn or other elongated material when heated, involves mounting a looped specimen of the yarn between hooks in a small oven provided with a heater and a temperature sensor. One hook is attached to a tension gage having an electrical output indicative of load or force and the other is positioned at a distance which permits a taut loop. The oven temperature is then increased at a programmed rate and the temperature in the oven measured and fiber tension or stress plotted on a graph as a function of temperature. As is known, many yarns in the lower temperature ranges show single or multiple peaks in the tension/temperature plot. The location and magnitude of the peak(s) characterizes the yarn and its mechanical and thermal history.
Another desirable measurement is that of stress relaxation measurement of plastics. A standard test for such measurement is described in ASTM specification D-2991-71. According to this test, a piece of plastic is placed between two gripping members and the time dependent change in the stress which results from the application of a constant total strain to the specimen at a constant temperature is obtained. The electrical outputs of a load cell and extensometer are recorded as a function of time and the stress calculated which is then plotted as a function of time.
A disadvantage of all of these prior art techniques involves the use of an active load cell as well as the use of an extensometer for measuring displacement of the sample under load conditions whether generated internally or externally. This requires the utilization of specific instrumentation apparatus having in most instances only a single use. Furthermore, load cells (particular double element cells) are in many cases subject to failure in that the use of one more active element increases the chances of such failure. Also such cells often tend to be noisy and increase the measurement error.
A thermomechanical analyzer is described in U.S. Pat. No. 3,474,658, issued Oct. 28, 1969 to Levy et al. There is described in the said Levy patent a thermomechanical analyzer capable of accurately measuring displacement of material samples under various temperature and load conditions. This analyzer uses a vertical shaft slideably mounted therein and has a probe attached to the shaft at one end to act on the sample. Since the Levy et al. analyzer is a highly sensitive and accurate unit, it would be highly desirable to provide it with an attachment such that it could measure stress/tension as well.
Accordingly, it is the object of this invention to provide an improved stress measuring device.
Another object of this invention is to provide an improved stress measuring method.
A further object of this invention is to provide an improved method of converting a displacement transducer into a stress measuring device.
An additional object of this invention is to provide an improved stress-tension measuring device utilizing a passive element.